Apparatus and methods for determining the minority carrier diffusion length (L) using the SPV method are well known. In brief, the principle of the diffusion length (L) determination requires the illumination of a specimen surface with monochromatic radiation of energy slightly greater than the bandgap of the semiconductor. Electron-hole pairs are produced and diffuse to the illuminated (front) surface where they are separated by the electric field of the depletion region (i.e., the surface-space-charge region) to produce a surface photovoltage (SPV). A portion of the SPV signal is coupled to an amplifier for amplification and measurement. The photon intensity (photons per second) is adjusted to produce the same magnitude of SPV at various wavelengths of illumination. The photon intensity required to produce this constant magnitude SPV signal is conveniently plotted on the ordinate against the reciprocal of the absorption coefficient on the abscissa for each wavelength. The resultant plot is linear and is extrapolated to the zero intensity intercept on the negative abscissa. This intercept value is the effective diffusion length (L). For a more detailed description of the theory and background for this method, see my article "A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors," in the Journal of Applied Physics, Vol. 32, No. 12, pp. 2550-2552, December 1961. The American Society for Testing and Materials has adopted a standard using this method which is published as ASTM F 291-78. The ASTM standard, when implemented according to the block diagram of FIG. 1 of ASTM F 291-78, is provided particularly for testing the diffusion length (L) for minority carriers in silicon but the method may be used for other semiconductor materials. This ASTM method will be discussed further hereinafter.
Equipment is available for carrying out the measurements to determine the diffusion length described above. One such equipment is manufactured by Solar Semiconductor Instruments, of Garland, Texas as the SSI Model 100 diffusion length meter. However, the known equipments are deficient in one or more ways that make their use difficult and time consuming. Each of these known equipments requires manual wavelength selection such as using a manually operated monochromator or a filter wheel and the manual adjustment of the light level by the operator at each wavelength to achieve a constant-magnitude SPV. Another problem is the low signal-to-noise (SNR) ratio of the surface photovoltage that is produced by such equipment.
In the pickup arrangement used in the prior art, the minority carriers generated in the illuminated region of the sample slowly diffuse to the nonilluminated region of the surface and there change the surface properties. The capacitance of the nonilluminated region of the surface is slowly changed thereby. The change in capacitance presents a time-varying attenuation to the ac portion of the generated surface photovoltage (SPV) that is coupled to an amplifier, thereby producing the observation of drift of the measured portion of the surface photovoltage with time. This drift effect results in making difficult the determination of the minority carrier diffusion length (L). Significant drifting during the course of the measurements will make it impossible to determine L.